The Hell is concentrated on getting rid of all the initial bugs of Diablo 1, it raises challenge, gives much more variety (monsters, items, spells, boss monsters, unique items, item & environment colors), improves overall difficulty balance of the game, improves monster AI, introduces new character classes, increases replay value and gives full support for Multiplayer experience.
It is fully functional, has many bugs of original game fixed, is running smooth, stable, and is much better than original game.
The amount of changes is tremendous, part of them are available in Changelog file inside the game archive.